1. Field of the Invention
The present invention relates to a mobile radio communication system, and more particularly to an apparatus for and a method of antenna-switched diversity reception.
2. Description of the Related Art
Antenna diversity reception schemes are roughly classified into a post-detection selection diversity system and an antenna-switched diversity system. In the post-detection selection diversity system, antenna branches are associated with respective demodulators, and a demodulated signal from the antenna branch which represents the best signal reception is selected. While the post-detection selection diversity system is capable of good signal reception in fading environments, the circuit used by the system is relatively large in scale, expensive, and consumes a large amount of electric power because each of the antenna branches needs to be associated with a demodulator. There have been proposed various attempts to eliminate the above problems. For example, reference should be made to Japanese patent application laid-open No. 6-268635.
The antenna-switched diversity reception system switches between a plurality of antennas located in spatially different positions to select one of the antennas which provides a good reception quality. Since only the antennas are switched, the antenna-switched diversity reception system can easily meet the requirements for rendering communication apparatus small, lightweight, and low-power-consumption. However, the quality of received signals remains to be improved, and various efforts have been made to receive high-quality signals.
For example, Japanese patent application laid-open No. 5-206905 discloses an antenna-selected diversity receiver which measures reception powers of two antennas, twice for each antenna, immediately before a burst signal is received, linearly predicts reception powers of the antennas for the burst signal from the measured values, and stores the predicted values in a memory. For receiving the burst signal, one of the antennas whose predicted value is higher is selected.
Furthermore, using the reception power of the antenna which is being selected, a subsequent reception power is predicted, and the data stored in the memory is updated by the predicted value.
An antenna-selected diversity reception apparatus disclosed in Japanese patent application laid-open No. 7-235901 has a holding means for holding the reception levels of two antennas. When a certain signal inserted in a received signal is reached, one of the antennas whose reception level is higher is selected.
According to each of the above proposals, an antenna whose reception level is higher is selected to ensure a desired level of reception quality by switching between antennas even while a burst signal is being received.
With the antenna-selected diversity receiver which employs the linear predict for the reception powers in advance, since the switching between the antennas is controlled by the predicted values, the antennas may not be properly switched, resulting in a relatively high probability that a reception error occurs.
Furthermore, the antenna-selected diversity receiver needs a control system for performing predictive calculations. Such predictive calculations causes a large burden on the processor of the control system, and require the control system to have an increased circuit scale and consume an increased power.
The antenna-selected diversity reception apparatus which triggers the switching between the antennas with a certain signal inserted in a received signal is unable to switch between the antennas until the certain signal is received even when the reception state is impaired. Consequently, the antenna-selected diversity reception apparatus tends to cause a reception error.